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Ublas :

From: Tan, Sarah \(FID\) (Sarah.Tan_at_[hidden])
Date: 2007-09-24 10:02:09


Hi
 
I am wondering whether anyone has experience with the stability issue
when compiling ublas with Intel C++ Compiler for Windows (ICC 9.1).
 
Problem arises when parallel code is generated (boost library 1.33.1).
The parallelization scheme is quite simple. There are four threads for a
four-core machine. Each thread dumps output to a ublas matrix. They then
got accumulated into a final matrix when four threads all come back.
 
ublas::matrix<double> output(n1, n2);

output.clear();

std::vector< ublas::matrix<double> > outputVec;

 for (unsigned i = 0; i < 4; ++i)

{

ublas::matrix<double> um(n1, n2);

      um.clear();

      outputVec.push_back(um);

}

#pragma omp parallel

{

#pragma omp sections

{

#pragma omp section

{ outputVec[0] = doSomething();}

#pragma omp section

{ outputVec[1] = doSomething();}

#pragma omp section

{ outputVec[2] = doSomething();}

#pragma omp section

{ outputVec[3] = doSomething();}

}

#pragma omp barrier

for (unsigned i = 0; i < 4; ++i)

      output += outputVec[i];

 
If ICC "Generate parallel code /QOpenmp" option is selected, output is
unstable. If ICC "Generate sequential code /QOpenmp-stubs" option is
selected, output is STABLE, but of course I no longer enjoy the speed
boost because omp directives are ignored.
 
The whole parallelization scheme is perfectly stable if std::vector
based matrix is used rather than ublas, which is considerably slower
than ublas.
 
How can I get a stable paralyzed ublas-based implementation? Any input
is much appreciated.
Thanks
Sarah
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